Webs formed from absorbent nonwoven pulp fibers have long been used as practical and convenient disposable hand towels or wipes. These nonwoven webs are typically manufactured by conventional high speed papermaking processes having additional post-treatment steps designed to increase the absorbency or other characteristics of the web. Exemplary post-treatment steps include creping, aperturing, embossing, hydraulic needling, hydroentanglement, binder addition, and the like. Most web-forming processes use either a wet-laid process or an air-laid process. A wet-laid process deposits a slurry of fibers in water onto a moving foraminous support surface, typically a wire mesh, screen or fabric, using water flow to lay down the fibers. The fibers are thus oriented predominantly in the x,y-directions. Webs created by a wet-laid process are ordinarily less expensive to produce than by an air-laid process, but the wet-laid web has poorer z-direction fiber orientation. Thus, paper, such as typing paper, has good x,y-direction tensile strength characteristics, but poor softness, bulk, absorptivity and z-direction thickness. For absorbent products, such as wipes, softness, thickness, strength and absorbency during use are key desired qualities.
Many of the items or products into which wet-laid web materials are incorporated are generally regarded as being limited-use disposable products. By this it is meant that the product or products are used only a limited number of times and in some cases only once before being discarded. With increasing concerns over solid waste disposal, there is now an increasing need for materials that are, for example, either recyclable or disposable through other mechanisms besides incorporation into landfills. One possible alternative means of disposal for many products, especially in the area of personal care absorbent products and wipers, is by flushing them into sewage disposal systems. As will be discussed in greater detail below, flushable means that the material must not only be able to pass through a commode without clogging it, but that the material must also be able to pass through the sewer laterals between a house (or other structure housing the commode) and the main sewer system without getting caught in the piping, and to disperse into small pieces that will not clog a toilet or the sewer transport and treatment process.
In recent years, more sophisticated approaches have been devised to impart dispersibility. Chemical binders that are either emulsion or melt processable or aqueous dispersions have been developed. Such chemical binders are typically sprayed or printed onto the web and absorbed or partially absorbed by the fibers. The material can have high strength in its original storage environment, but quickly lose strength by debonding or dispersing when placed in a different chemical environment (e.g., pH or ion concentration), such as by flushing down a commode with fresh water. It would be desirable to have a bonding system that would produce a fabric having desirable strength characteristics, yet be able to rapidly disperse or degrade after use into small pieces.
U.S. Pat. No. 4,309,469 and 4,419,403, both issued to Varona, describe a dispersible binder of several parts. Reissue Patent No. 31,825 describes a twostage heating process (preheat by infrared) to calendar bond a nonwoven consisting of thermoplastic fibers. Although offering some flexibility, this is still a single thermal bonding system. U.S. Pat. No. 4,207,367 issued to Baker, describes a nonwoven which is densified at individual areas by cold embossing. The chemical binders are sprayed on and the binders preferentially migrate to the densified areas by capillary action. The non-densified areas have higher loft and remain highly absorbent. However, it is not a hybrid bonding system because the densification step is not strictly a bonding process. U.S. Pat. No. 4,749,423, issued to Vaalburg et al., describes a two stage thermal bonding system. In the first stage, up to 7% of the polyethylene fibers in a web are fused to provide temporary strength to support transfer to the next processing stage. In the second stage the primary fibers are thermally bonded to give the web its overall integrity. This process in two distinct stages does not permit the web to have a structure of built-in areas of strength and weakness. It is not suitable as a dispersible material.
Several patents describe hybrid bonding systems, but are for sanitary napkin covers. For example, see U.S. Pat. No. 3,654,924, to Duchane, U.S. Pat. No. 3,616,797, issued to Champagne et al., and U.S. Pat. No. 3,913,574, issued to Srinvasan et al. The important difference is that these products are designed to be stored dry and to have very limited wet strength for a short duration during use. In a wet wipe there remains a need for prolonged wet strength in a storage solution.
U.S. Pat. No. 5,137,600, issued to Barnes et al. and commonly assigned to the assignee of the present invention, describes a hydropoint process for improving z-direction orientation and thickness. U.S. Pat. No. 4,755,421, issued to Manning et al. describes a process for forming a hydroentangled disintegratable fabric. U.S. Pat. No. 5,508,101, issued to Patnode et al., discloses a web composed of a hydrolytically degradable polymer and a water soluble polymer, such that the material, when submersed in water at an elevated temperature and elevated pH, will disintegrate. This web material appears to be primarily used in a laundry cycle where such extreme conditions occur. It would be desirable to have a fabric article that is dispersible at room temperature and nominal pH conditions, such as those that exist in the common household toilet bowl. U.S. Pat. No. 5,292,581, issued to Viazmensky et al., discloses a wet wipe that has strength characteristics, but is not immediately dispersible in water.